The present invention relates to an apparatus for controlling the change ratio of a stepless transmission with belt drive system, and, more particularly, to an apparatus which improves fuel consumption efficiency by employing the stepless transmission in, for example, a vehicle.
As a so-called stepless transmission capable of adjusting the change ratio (to be designated by symbol ".gamma." hereinafter) in a stepless and variable manner, a structure has been disclosed, for example, in Japanese Patent Laid-Open No. 63-42146. A stepless transmission of the type described therein comprises a transmission including a drive pulley (to be called a "primary pulley" hereinafter) the effective radius of which can be varied, a follower pulley (to be called a "secondary pulley" hereinafter), and a belt arranged between the previously described two pulleys. Thus, the effective radii of the primary pulley and the secondary pulley are continuously adjusted by applying/discharging hydraulic pressure to the hydraulic cylinder of the drive pulley. When a stepless transmission of the type described above is applied to a vehicle, the primary pulley is connected to the engine side (usually it is connected to the forward/reverse switch mechanism) so that the engine output is transmitted through the secondary pulley. Furthermore, the change ratio of the stepless transmission is, for example, defined to be the ratio of the rotational speed of the secondary pulley with respect to the engine speed.
In a conventional transmission which employs a multi-step gear, the change ratio .gamma. is varied in a stepped manner. However, in a stepless transmission, the change ratio is varied continuously. Theoretically, if the engine speed N.sub.E is to be a constant speed, the vehicle speed V is continuously changed by simply continuously changing the change ratio .gamma.. If a driver operates the acceleration pedal to raise the vehicle speed, the vehicle speed can be increased by reducing the change ratio .gamma. without enlarging the throttle opening degree if there is excess engine torque. If there is no excess engine torque, the desire of a driver can be met by controlling the throttle opening degree. That is, in the case of an electronically controllable throttle, the change ratio and the throttle opening degree are controlled in accordance with the accelerator opening degree and the present vehicle speed V. In the case of a mechanical throttle in which the accelerator operation directly corresponds to the throttle opening degree, the change ratio is the subject to be controlled for the purpose of the desired vehicle speed.
The continuous variation of the change ratio .gamma. of the stepless transmission involves an upper limit (to be called an "upper limit change ratio") and a lower limit (to be called a "lower limit change ratio") depending upon physical restrictions due to the shape of each of the two pulleys. Therefore, in the stepless transmission, the change ratio is varied between the previously described upper limit change ratio .gamma..sub.max and the lower limit change ratio .gamma..sub.min, which have been stationarily determined in accordance with the mechanical structure of the pulleys. The reason for the existence of the limits of the change ratio lies in physical limitations in the size of the pulleys.
Therefore, the stepless transmission for a vehicle must be arranged in a manner such that the control for realizing a desired vehicle speed is performed such that the corresponding change ratio does not exceed the previously described upper limit change ratio or lower limit change ratio, as well as the previously described control of the change ratio.
For example, a normal operation control is performed in a manner such that the change ratio is continuously controlled, and the throttle opening degree is controlled as well if the change ratio exceeds the previously described two limits so that the constant vehicle speed is maintained.
In a mode in which the stepless transmission is directly connected to the engine of a vehicle, it is preferable that the change ratio be restricted to a low level so that engine speed can be maintained at low speed, and the fuel consumption efficiency can thereby be improved. Therefore, the lower the change ratio is, the more fuel consumption efficiency is improved.
However, the inventors of the present invention have discovered the fact that the fuel consumption efficiency of an actual vehicle cannot be improved by simply restricting the change ratio to the lowest level.
The problem above will be described with reference to FIGS. 1 and 2. FIG. 1 is a graph which illustrates the relationship between the transmission efficiency with respect to the change ratio .gamma. of a stepless transmission. Transmission efficiency means the efficiency of the transmission mechanism with regard to power in to the transmission as compared to the power out of the transmission. FIG. 2 is a graph which illustrates the losses and change in various resistances (hereinafter collectively identified as "RES") at each portion of a stepless transmission with respect to the revolution speed N.sub.P (that is, the input revolution speed of the transmission) of the primary pulley.
As shown in FIG. 1, the transmission efficiency TR of the transmission mechanism of a stepless transmission is varied in accordance with the change ratio .gamma.. Specifically, the transmission efficiency TR is reduced in proportion to the chance ratio .gamma.. FIG. 2 illustrates the reason why the transmission efficiency deteriorates as shown in FIG. 1. Referring to FIG. 2, line I designates the power loss of the oil pump, line II designates the dragging resistance of the clutch, line III designates the resistance generated due to the revolution of the primary pulley, line IV designates the overall resistance at the maximum change ratio, and line V designates the overall resistance at the lower limit change ratio. Therefore, symbol VI designates the quantity of the increase of the line IV with respect to the line III, VII designates the quantity of the increase of the line V with respect to the line III, that is, the increase in the resistance at the primary shaft at the lower limit change ratio is shown in FIG. 2. As is shown from a comparison made between lines IV and V, the rotational resistance of the power transmission system in front of the secondary pulley varies in inverse proportion to the change ratio. That is, the rotational resistance increases in inverse proportion to the change ratio. The lower the change ratio becomes, the larger the resistance at the primary shaft. As a result, the overall rotational resistance of the power transmission system increases. Therefore, the lower the change ratio becomes, the more the transmission efficiency deteriorates, as is shown in FIG. 1.
When the change ratio is set to a value which approximates the minimum (lower limit change ratio) value in a low load region, a problem arises in that the fuel consumption efficiency deteriorates due to deterioration in the transmission efficiency of the transmission. However, a balance point exists at which the reduction in the fuel consumption efficiency due to the restriction of engine speed and the decrease in the fuel efficiency due to increased resistance caused by a reduction in the change ratio are able to balance each other. Therefore, control must be performed in a manner such that the change ratio does not exceed the previously described balance point for the purpose of improving fuel consumption efficiency.